/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
package org.skfiy.util.v8;

import java.io.Serializable;

/**
 * One or more variables that together maintain an initially zero {@code double}
 * sum. When updates (method {@link #add}) are contended across threads, the set
 * of variables may grow dynamically to reduce contention. Method {@link #sum}
 * (or, equivalently {@link
 * #doubleValue}) returns the current total combined across the variables
 * maintaining the sum.
 *
 * <p>This class extends {@link Number}, but does <em>not</em> define methods
 * such as {@code equals}, {@code hashCode} and {@code
 * compareTo} because instances are expected to be mutated, and so are not
 * useful as collection keys.
 *
 * <p><em>jsr166e note: This class is targeted to be placed in
 * java.util.concurrent.atomic.</em>
 *
 * @since 1.8
 * @author Doug Lea
 */
public class DoubleAdder extends Striped64 implements Serializable {

    private static final long serialVersionUID = 7249069246863182397L;

    /**
     * Update function. Note that we must use "long" for underlying
     * representations, because there is no compareAndSet for double, due to the
     * fact that the bitwise equals used in any CAS implementation is not the
     * same as double-precision equals. However, we use CAS only to detect and
     * alleviate contention, for which bitwise equals works best anyway. In
     * principle, the long/double conversions used here should be essentially
     * free on most platforms since they just re-interpret bits.
     *
     * Similar conversions are used in other methods.
     */
    final long fn(long v, long x) {
        return Double.doubleToRawLongBits(Double.longBitsToDouble(v)
                + Double.longBitsToDouble(x));
    }

    /**
     * Creates a new adder with initial sum of zero.
     */
    public DoubleAdder() {
    }

    /**
     * Adds the given value.
     *
     * @param x the value to add
     */
    public void add(double x) {
        Cell[] as;
        long b, v;
        HashCode hc;
        Cell a;
        int n;
        if ((as = cells) != null
                || !casBase(b = base,
                Double.doubleToRawLongBits(Double.longBitsToDouble(b) + x))) {
            boolean uncontended = true;
            int h = (hc = threadHashCode.get()).code;
            if (as == null || (n = as.length) < 1
                    || (a = as[(n - 1) & h]) == null
                    || !(uncontended = a.cas(v = a.value,
                    Double.doubleToRawLongBits(Double.longBitsToDouble(v) + x)))) {
                retryUpdate(Double.doubleToRawLongBits(x), hc, uncontended);
            }
        }
    }

    /**
     * Returns the current sum. The returned value is <em>NOT</em> an atomic
     * snapshot; invocation in the absence of concurrent updates returns an
     * accurate result, but concurrent updates that occur while the sum is being
     * calculated might not be incorporated. Also, because floating-point
     * arithmetic is not strictly associative, the returned result need not be
     * identical to the value that would be obtained in a sequential series of
     * updates to a single variable.
     *
     * @return the sum
     */
    public double sum() {
        Cell[] as = cells;
        double sum = Double.longBitsToDouble(base);
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null) {
                    sum += Double.longBitsToDouble(a.value);
                }
            }
        }
        return sum;
    }

    /**
     * Resets variables maintaining the sum to zero. This method may be a useful
     * alternative to creating a new adder, but is only effective if there are
     * no concurrent updates. Because this method is intrinsically racy, it
     * should only be used when it is known that no threads are concurrently
     * updating.
     */
    public void reset() {
        internalReset(0L);
    }

    /**
     * Equivalent in effect to {@link #sum} followed by {@link
     * #reset}. This method may apply for example during quiescent points
     * between multithreaded computations. If there are updates concurrent with
     * this method, the returned value is
     * <em>not</em> guaranteed to be the final value occurring before the reset.
     *
     * @return the sum
     */
    public double sumThenReset() {
        Cell[] as = cells;
        double sum = Double.longBitsToDouble(base);
        base = 0L;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null) {
                    long v = a.value;
                    a.value = 0L;
                    sum += Double.longBitsToDouble(v);
                }
            }
        }
        return sum;
    }

    /**
     * Returns the String representation of the {@link #sum}.
     *
     * @return the String representation of the {@link #sum}
     */
    public String toString() {
        return Double.toString(sum());
    }

    /**
     * Equivalent to {@link #sum}.
     *
     * @return the sum
     */
    public double doubleValue() {
        return sum();
    }

    /**
     * Returns the {@link #sum} as a {@code long} after a narrowing primitive
     * conversion.
     */
    public long longValue() {
        return (long) sum();
    }

    /**
     * Returns the {@link #sum} as an {@code int} after a narrowing primitive
     * conversion.
     */
    public int intValue() {
        return (int) sum();
    }

    /**
     * Returns the {@link #sum} as a {@code float} after a narrowing primitive
     * conversion.
     */
    public float floatValue() {
        return (float) sum();
    }

    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        s.defaultWriteObject();
        s.writeDouble(sum());
    }

    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        busy = 0;
        cells = null;
        base = Double.doubleToRawLongBits(s.readDouble());
    }
}
